Ziegler-Natta catalysts are generally composed of a catalyst support material and a transition metal component. The transition metal component is typically a substituted Group 4-8 transition metal, with titanium, zirconium, chromium or vanadium being commonly used. The transition metal is often provided as a metal halide, such as TiCl4. Ziegler-Natta catalysts are used to effectively promote the high yield polymerization of olefins. In the polymerization of olefins, the catalyst is often used in combination with an organoaluminum cocatalyst.
When used to catalyze polymerization of propylene, a third component has to be used in the catalyst. The third component is an electron donor used to control the stereoregularity of the polymer. It can be either incorporated into the catalyst during its synthesis (an internal donor), or it can be added to the polymerization reactor during the polymerization reaction (an external donor). In most polymerizations, both an internal donor and an external donor may be used. Various aromatic esters, diethers, succinates, alkoxysilanes and hindered amines are examples of compounds that have been used as internal donors.
One well known support material used in Ziegler-Natta catalysts is MgCl2. The MgCl2 material is sometimes complexed with ethanol (EtOH). In preparing the catalyst, typically most or all of the EtOH reacts with the transition metal halide, such as TiCl4.
Methods of producing MgCl2-xEtOH complexes, where x is the average number of EtOH molecules in the support material, are described in several patents. For example, U.S. Pat. No. 5,468,698 to Koshinen describes methods for preparing a MgCl2-xEtOH support material. A molten MgCl2-xEtOH complex (x=3.3 to 5.5) is sprayed into a heated chamber to form a particulate MgCl2-xEtOH material in which x=2.0 to 3.2. Koskinen does not describe the composition of any particular catalyst made using the support material.
Catalysts utilizing MgCl2-xEtOH supports are also described. For example, U.S. Pat. No. 4,829,034 to Iiskolan describes a Ziegler-Natta catalyst, and a method for making the catalyst, using a MgCl2-xEtOH support in which x is about 3. In Iiskolan, the support material is first contacted with an internal donor, such as D-i-BP. The support D-i-BP complex is then combined with TiCl4 to form the catalyst.
U.S. Pat. No. 6,020,279 to Uwai describes a method for making a Ziegler-Natta catalyst by producing a MgCl2-xEtOH support in which x=1.5 to 2.1 and the support has an average particle diameter of 91 μm. The support is combined with a titanium halide, such as TiCl4, and an internal electron donor for 10 minutes to 10 hours at 120° C. to 135° C. in the presence of an aliphatic solvent. As internal donors, esters like di-isobutyl-phthalate (Examples) are preferred.
While a variety of Ziegler-Natta catalysts have been developed, due to the importance of olefin polymerizations, there remains a need to develop catalysts having improved activity. Improving the activity of the catalyst leads to higher product yields and reduces the quantity of the catalyst required for the olefin polymerization reaction, which reduces the catalyst cost and the amount of catalyst impurities in the polymer (reduced ash content), resulting in polymers with a better performance profile.
Due to health, environment and safety concerns in connection with the use of phthalate containing Ziegler-Natta catalysts for the production of polymers with potential skin or food contact, a second driver to develop new Ziegler-Natta catalysts is the need to provide non-phthalate catalyst versions that produce polymers with an identical or at least very similar performance profile as the currently broadly used phthalate containing Ziegler-Natta catalysts.
Well known alternatives to Ziegler-Natta catalysts based on phthalates as internal donors are versions where various malonates, succinates or diether compounds are used. Unfortunately, the use of such alternative internal donors results in polymers with fully different performance profiles. As an example and a direct comparison, the use of a phthalate based Ziegler-Natta catalyst leads to polymers with a GPC Polydispersity Index (PI(GPC)) (also referred to as Molecular Weight Distribution or Mw/Mn) in the range of 6.5 to 8, when using certain diethers as an internal donor the polydispersity is much more narrow (4.5 to 5.5), and when using succinate as internal donor the polydispersity is 10 to 15 (Polypropylene Handbook, 2nd Edition, Editor: Nello Pasquini, Carl Hanser Verlag, Munich, 2005, page 18, Table 2.1 and P. Galli, G. Vecellio, Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 42, 396-415 (2004), pages 404-405 and Table 1).
The molecular weight distribution is one of the most important properties of a polymer. By changing this parameter, the crystalline structure and the crystallization rate of a polymer is dramatically influenced, which has impact on the convertability and usability of a certain polymer for a certain application. As an example, for extrusion applications like sheet, pipe, film, raffia, or thermoforming, a broader molecular weight distribution is advantageous, while for applications like fiber or injection molding a narrower molecular weight distribution would be advantageous. As used to processing polymers produced using phthalate based Ziegler-Natta catalysts, the converters insist in molecular weight distributions typically produced by such catalysts and expect that phthalate free Ziegler-Natta catalysts deliver a similar molecular weight distribution. Unfortunately, state of the art diether based catalysts deliver polymers where the molecular weight distribution is too narrow while succinate based catalysts deliver polymers where the molecular weight distribution is far too broad.